1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing method thereof and more particularly to a stack multi-chip package (MCP) having a plurality of semiconductor chips stacked and packaged.
2. Description of the Related Art
Conventionally, a stack MCP is formed by performing the process for forming elements on a semiconductor wafer, grinding the backside of the wafer, attaching film-like adhesive to the backside, dicing the wafer to form semiconductor chips, stacking and mounting the chips in a plural-stage form by repeatedly performing the process for mounting the thus formed chips and the wire bonding process, and then resin-molding the thus stacked chips.
FIG. 1 is a cross-sectional view showing an enlarged portion of a conventional stack MCP. In this case, an example wherein four semiconductor chips are stacked and mounted is shown. A semiconductor chip 13-1 is mounted on a chip mounting portion of a printed circuit board 11 with an adhesive layer 12-1 disposed therebetween. A pad 14-1 of the chip 13-1 and a wiring layer 15-1 on the printed circuit board 11 are electrically connected to each other via a bonding wire 16-1.
A second semiconductor chip 13-2 smaller than the chip 13-1 is mounted on the first semiconductor chip 13-1 with an adhesive layer 12-2 disposed therebetween. A pad 14-2 of the chip 13-2 and a wiring layer 15-2 on the printed circuit board 11 are electrically connected to each other via a bonding wire 16-2.
A spacer 17 smaller than the chip 13-2 is formed on the second semiconductor chip 13-2 with an adhesive layer 12-3 disposed therebetween. As the spacer 17, for example, a chip (which is called a spacer chip) on which no element is formed can be used.
A semiconductor chip 13-3 having substantially the same size as the chip 13-1 is mounted on the spacer 17 with an adhesive layer 12-4 disposed therebetween. A pad 14-3 of the chip 13-3 and a wiring layer 15-3 on the printed circuit board 11 are electrically connected to each other via a bonding wire 16-3.
A semiconductor chip 13-4 having substantially the same size as the chip 13-2 is mounted on the semiconductor chip 13-3 with an adhesive layer 12-5 disposed therebetween. A pad 14-4 of the chip 13-4 and a wiring layer 15-4 on the printed circuit board 11 are electrically connected to each other via a bonding wire 16-4.
The chips 13-1 to 13-4 and spacer 17 mounted in a stack form and the bonding wires 16-1 to 16-4 are sealed into a resin package 18.
Multi-layered wiring (not shown) is formed in the printed circuit board (multi-layer printed circuit board) 11, and the wiring layers 15-1 to 15-4 formed on the chip mounting surface of the printed circuit board 11 are electrically connected to respective wiring layers 19-1 to 19-4 formed on the backside thereof. Solder balls 20-1 to 20-4 for external connection are formed in an array form on the wiring layers 19-1 to 19-4 (BGA).
However, the above stack MCP and the manufacturing method thereof have the following problems (a) to (f).
(a) Semiconductor chips are formed by attaching film-like adhesive onto the wafer backside after the backside of the semiconductor wafer is ground and dividing the wafer by dicing. Therefore, backside chipping occurs in the dicing process in many cases and only chips which are low in bending strength can be formed. Thus, it is only possible to form chips with a thickness of 100 to 150 μm or more (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 11-204720).
(b) As described in (a), since chipping often occurs on the backside of the thus formed semiconductor chip, there occurs a possibility that cracks will occur in the chip at the time of wire bonding onto the element forming surface.
(c) As a method for solving the problems of (a) and (b), it is considered that a dicing before grinding (DBG) method (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-17513) is used. However, if film-like adhesive is attached to the backsides of chips in order to mount the chips in a stack form after the wafer is divided by the dicing before grinding method, the divided chips will stick together and it becomes necessary to cut the film-like adhesive on the backside. The cutting process can be made unnecessary if film-like adhesive is attached to the individual chips. However, if film-like adhesive is individually attached to a large number of chips, the manufacturing process becomes complicated.
(d) Further, as another method for solving the problems of (a) and (b), a method for enhancing the bending strength of the chip by etching the ground surface to eliminate chipping after the backside of the semiconductor wafer is ground is considered. However, with this method, if the chip thickness is reduced to 100 μm or less, cracks or breakage larger than those occurring in a case where no etching process is performed may occur. As a result, there occurs a possibility that the number of faulty chips is increased.
(e) When the sizes of chips to be stacked are substantially the same, wire bonding is performed for the upper-side chip in a suspended state and the possibility that poor connection occurs becomes strong if the rigidity of a thin chip is low. For example, in FIG. 1, the wire bonding portion of the chip 13-3 has an overhung of ΔL from the end portion of the spacer 17 and the end portion of the chip 13-3 is ball-bonded in a suspended state (in a non-fixed state). At this time, the bonding position may be deviated due to warping of the chip 13-3 or cracks will occur due to pressure at the bonding time. Further, if cracks do not occur, there occurs a possibility that the chip 13-3 will be bent and brought into contact with the lower-side bonding wire 16-2, thereby causing poor connection.
(f) In order to alleviate problem (e), a method for eliminating chipping and warping by etching the ground surface after the backside of the semiconductor wafer is ground is considered. However, even if the problem of cracking and warping can be solved by the above method, a problem of poor connection caused by bending of the chip 13-3 by pressure at the ball bonding time cannot be solved.